The method for production of carbon fibers includes chemical vapor deposition methods (methods in which hydrocarbon and the like is thermally decomposed on catalyst metals to form carbon fibers), and physical vapor deposition methods (methods in which graphite is allowed to undergo sublimation by arc, laser or the like to form carbon fibers in a cooling process).
The chemical vapor deposition methods are methods suited for large-scale synthesis since it is comparatively easy to scale-up a reactor.
The chemical vapor deposition methods can be roughly classified into two methods. One is a method in which a solution, prepared by dissolving metal compounds serving as catalyst or co-catalyst such as sulfur in hydrocarbon such as benzene, is supplied to reaction field heated at 1,000° C. or higher using hydrogen as carrier gas, and formation of catalysts and growth of carbon fibers are performed in the field (floating catalyst method). The other one is a method in which supported catalyst (in which catalyst metals or precursors are supported on carrier) prepared in advance is placed in the reaction field heated at 500 to 700° C., and mixed gasses of hydrocarbon such as ethylene with hydrogen or nitrogen is supplied and then reacted (supported catalyst method).
Since the reaction is performed in high temperature range of 1,000° C. or higher in the floating catalyst method, not only decomposition of the hydrocarbon on the catalyst metals but also an self-decomposition reaction of hydrocarbon proceeds. Pyrolytic carbon is deposited on the carbon fibers grown from the catalyst metals as the starting point, and carbon also grows in the thickness direction of the fibers. The carbon fibers obtained by this method have comparatively low conductivity since they are coated with pyrolytic carbon having low crystallinity. Therefore, the carbon fibers are synthesized by the floating catalyst method, and then graphitized by heat treatment in an inert gas atmosphere at a temperature of 2,600° C. or higher. The heat treatment enables proceeding of crystal rearrangement and graphite crystal growth, leading to an improvement in conductivity of the fibers. The heat treatment also enables vaporization of the catalyst metals to give carbon fibers with less impurity.
On the other hand, since the reaction is performed at 500 to 800° C. in the supported catalyst method, the self-decomposition reaction of the hydrocarbon is suppressed. It is possible to obtain thin carbon fibers as a result of growing from the catalyst metals as the starting point. The obtained carbon fibers have comparatively high crystallinity and comparatively high conductivity. Therefore, it is not necessary to perform the heat treatment for graphitization which is applied to the carbon fibers obtained by the floating catalyst method. Since the carbon fibers synthesized by the supported catalyst method are not subjected to the heat treatment at high temperature for graphitization, the catalyst metals remain in a percentage order in the carbon fibers.